(1) Field of the Invention
The present invention generally relates to a phase change recording medium, and more particularly to a phase change recording medium which enables an existing optical disk apparatus, such as a DVD-ROM player, to reproduce data from the phase change recording medium in a manner similar to an existing DVD-ROM (digital video disk read-only memory).
(2) Description of the Related Art
An optical disk apparatus records a data signal, such as a video signal or an audio signal, to an optical disk and reproduces the data signal from the optical disk. In the optical disk, tracks are formed with alternate grooves and lands on a substrate of the optical disk.
In an existing optical disk apparatus, a laser beam from a laser light source is focused on one of the grooves and lands on the optical disk, and data is recorded to or reproduced from one of the grooves and the lands of the optical disk. The other of the grooves and the lands of the optical disk serve as a guard band which separates adjacent tracks from a recording track accessed by the optical disk apparatus. There also exists an optical disk apparatus adapted to record data to or reproduce data from both the grooves and the lands of the optical disk by focusing the laser beam on one of the grooves and the lands of the optical disk.
FIG. 5 shows a configuration of a conventional optical recording medium.
In FIG. 5, reference numeral 1 denotes a recording layer in the conventional optical recording medium, and the recording layer 1 is made of, for example, a phase change material. Reference numeral 3 denotes a light spot placed on the recording layer 1 of the recording medium. The recording layer 1 is provided with grooves 4 and lands 5, and data pits 2 and address pits 6 are provided on the lands 5 of the recording layer 1. The data pits 2 represent data recorded to the recording medium, and the address pits 6 represent positional data on the recording medium. In the conventional optical recording medium of FIG. 5, a transparent substrate of the recording medium is omitted.
FIG. 6 shows a conventional optical disk player/recorder apparatus which accesses an optical disk similar to the optical recording medium of FIG. 5.
In FIG. 6, reference numeral 10 denotes the optical disk, and reference numeral 11 denotes a track on the optical disk 10. The optical disk 10 is rotated by a spindle motor 9. In the conventional optical disk apparatus of FIG. 6, an optical head 18 is provided to access the optical disk 10.
In the optical head 18, a laser diode (LD) 12 is provided as a light source which emits a laser beam to the optical disk 10. A collimator lens 13 converts a laser beam emitted by the laser diode 12 into a parallel beam. A beam splitter 14 is provided in an optical path of the parallel beam from the collimator lens 13. An objective lens 15 converts the parallel beam passing through the beam splitter 14 into a converging beam so that the converging beam forms a light spot on the optical disk 10.
A reflection beam from the optical disk 10 passes through the objective lens 15, and it is reflected by the beam splitter 14 to a photodetector 16. The photodetector 16 is divided into two photodiodes 16a and 16b. The photodiodes 16a and 16b are provided such that both the photodiodes 16a and 16b extend in a direction parallel to a track of the optical disk 10. A tracking error (TE) signal is generated based on signals output from the photodiodes 16a and 16b. An actuator 17 supports the objective lens 15. The above-mentioned elements 12 through 17 are installed on a head base (not shown) of the optical head 18.
In the conventional optical disk apparatus of FIG. 6, a differential amplifier 21 is provided to generate a push-pull signal indicating a difference between the signals output from the photodiodes 16a and 16b. A low pass filter (LPF) 22 is provided to pass through a low-frequency component of the push-pull signal from the differential amplifier 21 and eliminate a high-frequency component of the push-pull signal. A tracking control unit 23 is provided to generate a tracking control signal based on the push-pull signal from the LPF 22 in response to a control signal "L1" from a system controller 30. The tracking control signal from the tracking control unit 23 is output to a driver unit 24. The driver unit 24 supplies a driving current to the actuator 17 in response to the tracking control signal from the tracking control unit 23, so that a position of the objective lens 15 relative to the optical disk 10 is controlled by the actuator 17.
In the conventional optical disk apparatus of FIG. 6, a summing amplifier 25 is provided to output a sum signal indicating a sum of the signals output from the photodiodes 16a and 16b. A high pass filter (HPF) 26 is provided to pass through a high-frequency component of the sum signal from the summing amplifier 26 and eliminate another low-frequency component of the sum signal. A playback signal processing unit 27 is provided to generate a data signal (such as an audio signal) based on the sum signal from the HPF 26. The sum signal provided at the input of the playback signal processing unit 27 is also called a playback signal. The data signal from the playback signal processing unit 27 is output to an output terminal "OUT". An address signal processing unit 28 is provided to generate an address signal based on the sum signal from the HPF 26. The address signal from the address signal processing unit 28 is output to the system controller 30.
In the conventional optical disk apparatus of FIG. 6, a traverse control unit 31 is provided to supply a driving current to a traverse motor 32 in accordance with a control signal "L2" from the system controller 30. The traverse motor 32 is provided to move the optical head 18 in a radial direction of the optical disk 10 in accordance with the driving current from the traverse control unit 31.
Further, in the conventional optical disk apparatus of FIG. 6, a recording signal processing unit 33 is provided to generate a recording signal based on a data signal (such as an audio signal) from an input terminal "IN". The recording signal from the recording signal processing unit 33 is supplied to an LD driver unit 34. The LD driver unit 34 is provided to supply a driving current to the LD 12 in accordance with the recording signal from the recording signal processing unit 33 and a control signal "L3" from the system controller 30. The system controller 30 inputs the address signal supplied from the address signal processing unit 28. The system controller 30 outputs the control signal "L1" to the tracking control unit 23. The system controller 30 outputs the control signal "L2" to the traverse control unit 31. The system controller 30 outputs the control signal "L3" to the LD driver unit 34.
An operation of the conventional optical disk apparatus of FIG. 6 will now be described.
The laser beam emitted by the LD 12 is converted into a parallel beam by the collimator lens 13. The parallel beam passing through the beam splitter 14 is converted into a converging beam by the objective lens 15, and the converging beam forms a light spot on the optical disk 10.
The reflection beam from the optical disk 10 passes through the objective lens 15. This reflection beam carries data according to the data pits 2 on the track of the optical disk 10. The reflection beam is reflected to the photodetector 16 by the beam splitter 14. The photodiodes 16a and 16b in the photodetector 16 convert the received reflection beam into electric signals. The signals from the photodiodes 16a and 16b are supplied to both the differential amplifier 21 and the summing amplifier 25.
The differential amplifier 21 generates a push-pull signal based on the difference between the signals output from the photodiodes 16a and 16b. The low pass filter (LPF) 22 passes through the low-frequency component of the push-pull signal from the differential amplifier 21, and outputs the push-pull signal to the tracking control unit 23 as a tracking error signal. The tracking control unit 23 generates a tracking control signal based on the control signal "L1" from the system controller 30 and the tracking error signal from the LPF 22. The tracking control signal from the tracking control unit 23 is output to the driver unit 24. The driver unit 24 supplies a driving current to the actuator 17 in response to the tracking control signal from the tracking control unit 23, so that a position of the objective lens 15 relative to the optical disk 10 is controlled by the actuator 17. The objective lens 15 is positioned by the actuator 17 in such a direction that the light spot crosses the track of the optical disk 10. The converging beam from the objective lens 15 is focused on the optical disk 10 by the positioning of the objective lens 15. The optical disk 10 is scanned by the optical head 18 while the light spot is moved along the track of the optical disk 10.
The objective lens 15 is positioned by a focusing control unit (not shown) in a direction perpendicular to the surface of the optical disk 10 such that the converging beam from the objective lens 15 correctly forms the light spot on the optical disk 10 at a focal distance of the objective lens 15.
The summing amplifier 25 generates a sum signal based on the sum of the signals output from the photodiodes 16a and 16b. The high pass filter (HPF) 26 passes through the high-frequency component of the sum signal from the summing amplifier 26, and outputs the sum signal to both the playback signal processing unit 27 and the address signal processing unit 28. The playback signal processing unit 27 generates a data signal based on the sum signal from the HPF 26. The data signal from the playback signal processing unit 27 is output to the output terminal "OUT". The address signal processing unit 28 generates an address signal based on the sum signal from the HPF 26. The address signal from the address signal processing unit 28 is output to the system controller 30.
The data signal is produced by the reflection beam as a result of the scanning of the light spot 3 to the data pits 2 of the optical disk as shown in FIG. 5. The address signal is produced by the reflection beam as a result of the scanning of the light spot 3 to the address pits 6 of the optical disk as shown in FIG. 5. The system controller 30 determines a position of the light spot 3 within the optical disk based on the address signal from the address signal processing unit 28.
The traverse control unit 31 supplies a driving current to the traverse motor 32 in accordance with the control signal "L2" from the system controller 30. The traverse motor 32 moves the optical head 18 to a designated track of the optical disk 10 in the radial direction of the optical disk 10 in accordance with the driving current from the traverse control unit 31. At this time, the tracking control unit 23 temporarily stops a tracking servo control for the optical head 18 in response to the control signal "L1" from the system controller 30. When the optical head 18 reproduces data from the optical disk 10, the traverse control unit 31 supplies a driving current to the traverse motor 32 in accordance with the tracking control signal from the tracking control unit 23. The optical head 18 is gradually moved in the radial direction of the optical disk 10 by the traverse control motor 32 in accordance with the progress of the reproduction of data.
When the optical head 18 records data in the optical disk 10, the recording signal processing unit 33 generates a recording signal based on a data signal (such as an audio signal) from the input terminal "IN", and supplies the recording signal to the LD driver unit 34. The system controller 30 outputs the control signal "L3" to the LD driver unit 34 so that the LD driver unit 34 is set in a recording mode. The LD driver unit 34 modulates the driving current in accordance with the recording signal from the recording signal processing unit 33, and supplies the modulated driving current to the LD 12. The intensity of the light spot on the optical disk 10 varies in accordance with the modulated driving current sent to the LD 12 so that the recording pits 2 are formed on the recording layer of the optical disk 10 by the laser beam from the LD 12.
When the optical head 18 reproduces data from the optical disk 10, the system controller 30 outputs the control signal "L3" to the LD driver unit 34 so that the LD driver unit 34 is set in a reproducing mode. The LD driver unit 34 maintains the driving current at a constant level, and supplies the constant level of the driving current to the LD 12. This enables the optical head 18 to detect the recording pits 2 and the address pits 6 on the optical disk 10. When the LD driver unit 34 is set in either the recording mode or the reproducing mode, the spindle motor 9 rotates the optical disk 10 at a constant linear velocity.
FIG. 7 shows a conventional tracking control circuit of an optical disk player apparatus.
In the conventional tracking control circuit of FIG. 7, a tracking control signal is generated based on a differential phase detection (DPD) tracking method. See Japanese Published Application No. 3-18255 for the DPD tracking method.
As shown in FIG. 7, a photodetector 35 is divided into four photodiodes S1, S2, S3 and S4. The photodiodes S1 and S3 convert the received reflection beam into electric signals. The signals from the photodiodes S1 and S3 are supplied to an adder 36. The adder 36 outputs a sum signal indicating a sum of the signals from the photodiodes S1 and S3. The photodiodes S2 and S4 convert the received reflection beam into electric signals. The signals from the photodiodes S2 and S4 are supplied to an adder 37. The adder 37 outputs a sum signal indicating a sum of the signals from the photodiodes S2 and S4.
In the tracking control circuit of FIG. 7, an adder 38 outputs a sum signal "Hf" (=(S1+S3)+(S2+S4)) indicating a sum of the sum signals from the adders 36 and 37. A subtractor 39 outputs a difference signal "D1" (=(S2+S4)-(S1+S3)) indicating a difference between the sum signals from the adders 36 and 37.
In the tracking control circuit of FIG. 7, the sum signal "Hf" from the adder 38 is supplied to each of a zero-crossing detection unit 41 and a zero-crossing detection unit 42. The difference signal "D1" from the subtractor 39 is supplied to each of a sample hold unit 43 and a sample hold unit 44. An output of the zero-crossing detection unit 41 is connected to an input of the sample hold unit 44. An output of the zero-crossing detection unit 42 is connected to an input of the sample hold unit 43.
FIG. 8 shows an operation of the conventional tracking control circuit of FIG. 7.
In FIG. 8, (a) indicates a waveform of the difference signal "D1" at the output of the subtractor 39, and (b) indicates a waveform of the sum signal "D1" at the output of the adder 38.
In the tracking control circuit of FIG. 7, the zero-crossing detection unit 41 outputs a sampling pulse to the sample hold unit 44 when the sum signal "H1" is on the increase and crosses zero, as indicated by (c) in FIG. 8. The zero-crossing detection unit 42 outputs a sampling pulse to the sample hold unit 43 when the sum signal "H1" is on the decrease and crosses zero, as indicated by (d) in FIG. 8. In the sample hold unit 44, the difference signal "D1" is sampled and held by using the sampling pulse from the zero-crossing detection unit 41. In FIG. 8, (e) indicates a waveform of a signal at the output of the sample hold unit 44. Further, in the sample hold unit 43, the difference signal "D1" is sampled and held by using the sampling pulse from the zero-crossing detection unit 42. In FIG. 8, (f) indicates a waveform of a signal at the output of the sample hold unit 43.
As shown in FIG. 8, the phase of the output signal (e) of the sample hold unit 44 and the phase of the output signal (f) of the sample hold unit 43 are substantially opposite to each other. A differential amplifier 45 of the tracking control circuit of FIG. 7 receives the output signals (e) and (f) from the sample hold units 44 and 43 and generates a tracking control signal based on a difference between the output signals (e) and (f).
Hereinafter, the tracking control signal output by the tracking control circuit of FIG. 7 is called a DPD signal. In the tracking control circuit of FIG. 7, a disturbance, such as an offset of the reflection beams received by the photodiodes S1-S4, can be eliminated from the tracking control signal. It is possible for the conventional tracking control circuit of FIG. 7 to output an accurate tracking control signal.
Further, Japanese Published Patent Application Nos. 56-30610 and 2-56734 disclose differential phase detection tracking methods which are similar to the above-described DPD tracking method.
In the DPD tracking method of the above-mentioned publications, a tracking servo control for an optical head is performed. A tracking error signal indicating an error of a light spot from a center of data pits on a track of an optical disk is obtained by detecting a change of an optical intensity distribution of reflection beams received by photodiodes of the optical head. The tracking servo control for the optical head is performed such that the tracking error signal is minimized.
Further, there exists a phase change recording medium in which a crystal phase of a recording layer is changed through a heating and cooling control while emitting a laser beam to the recording layer, so that data is recorded in the recording layer. In the recording layer of the existing phase change recording medium, a first phase which provides a large reflection factor and a second phase which provides a small reflection factor are alternately formed by the laser beam irradiation so that data is optically recorded in the recording layer. Data pits which correspond to the data pits 2 of the optical disk of FIG. 5 are formed in the recording layer by changing the crystal phase to the second phase, and areas of the recording layer where no data pit is formed remains in the first phase.
It is difficult for an existing DVD-ROM player to properly reproduce data from an existing phase change recording medium in a manner similar to an existing DVD-ROM.
The existing phase change recording medium has physical characteristics that provide a reflection factor and an amplitude modulation factor which are less than corresponding factors provided by the DVD-ROM. Because of this, the existing phase change recording medium fails to provide an adequate level of signal intensity for a DPD signal. It is difficult that the DVD-ROM player reproduces data from the existing phase change recording medium in a manner similar to the DVD-ROM. Since the existing phase change recording medium fails to provide an adequate level of signal intensity for a tracking control signal, it is difficult to effectively perform a tracking servo control based on the DPD tracking method. In order to properly reproduce data from the existing phase change recording medium, it is necessary to perform a tracking servo control based on a push-pull method for the conventional optical recording medium.